Measurement of biological tissue surface contour or layer thickness can provide useful diagnostic information in various applications. For example, arterial plaque thickness is related to the progress of atherosclerosis, carotid vessel wall thickness is also an indicator of cardiovascular disease risk; epidermal layer thickness is an indicator of burn severity.
In ophthalmology, retinal thickness may be abnormally large in cases of retinal edema or traction by membranes in the vitreous humor. On the other hand, the retina may appear thin in cases of atrophic degeneration, chorioretinitis, or trauma to the retina. Meanwhile, changes in retinal thickness may be localized or extend over large areas. In certain cases, the overall contour of the retina may become abnormal. For example, pronounced myopia, particularly due to posterior staphylomas, may create a highly concave retina. Detachment of the retinal pigment epithelium (RPE), subretinal cysts, or subretinal tumors may produce a relative convexity of the retina. Therefore, mapping the retina contour or retinal thickness makes it possible to determine the extent and severity of such conditions and to monitor progress of treatment.
In the past, there are a number of well-established biomedical imaging techniques that have been used for three-dimensional anatomical mapping of the eye, especially the retina, including optical coherence tomography (Zhou, Q. et al. (2004). “Mapping retinal thickness and macular edema by high-speed three-dimensional optical coherence tomography”. Ophthalmic Technologies XIV, SPIE, 5314: 119-125), ultrasound (see for example, U.S. Pat. Nos. 5,293,871, 5,562,095), and confocal microscopy (see for example, U.S. Pat. No. 4,838,679; R. H. Webb (1996) “Confocal optical microscopy” Rep. Prog. Phys. 59 427-471). The three-dimensional data set has also been analyzed to identify layered structures in the tissue using a variety of approaches to image segmentation. (see for example, D. G. Bartsch, et al., (2004) “Optical coherence tomography: interpretation artifacts and new algorithm”, Proc. SPIE Medical Imaging 2004: Image Processing, 5370: 2140-2151; H. Ishikawa, et al., (2005) “Macular Segmentation with Optical Coherence Tomography”. Invest Ophthalmol Vis Sci.; 46: 2012-201).
These prior art methods measured and/or generated a map of a tissue layer thickness by searching for the borders of the tissue layer structures, figuring out the inner and outer boundaries and then finding the distance between the inner and outer boundaries. However, a major issue associated with a tissue layer thickness map is that it sometimes cannot reveal the diagnostically more meaningful features of a diseased part of the tissue. For example, retina thickness is defined as the vertical distance between the RPE (retinal pigment epithelium) 102 and the ILM (inner limiting membrane) 104 as shown in FIG. 1. A sharp bump 106 of the retina will often be associated with a rise in the RPE 102 as well as the formation of a lesion 108 below the RPE 102, such that the RPE also has a broad rise. As a result, a retina thickness map such as the color coded one shown in FIG. 2, which corresponds to FIG. 1, cannot reveal the substantially raised bump. In fact, the color coded thickness map shows that the thickness will only slightly increase near the bump but then return to normal over it. On the other hand, although a topographic map or contour of the RPE or ILM may reveal the sharp bump better for this illustrated case than the retina thickness map, it would include both the sharp bump and the broader warping of the RPE boundary, making it difficult to separate the effect of the disease from the overall shape of the RPE boundary
In light of the above, there is a need in the art for a method for generating elevation maps with respect to a reference fitted surface and for using the reference surface as a means of locating three-dimensionally a tissue or a layer or boundary of a tissue such as the retina, in order to provide diagnostically more meaningful information about potential diseased tissue.
The present invention is a novel and non-obvious method wherein a fitted reference surface is used to create an elevation map or image of a tissue layer/boundary with respect to the fitted reference surface. Use of such a fitted surface can minimize the perturbations of the surface associated with disease so as to approximate the tissue surface that would exist if the tissue were normal. By using such a fitted surface, either of the tissue boundary being measured, or a different boundary, the effect of disease or injury is isolated from the overall shape of the tissue of interest, providing improved diagnostic information. In addition to various ways to display the elevation data relative to the fitted reference surface, the invention also combines the elevation data with other maps or images in order to provide more meaningful information for diagnostics.